Despite the almost universal use of diagnostic ultrasound in obstetrical patients in the United States, critical information regarding in situ fetal ultrasonic exposure levels, the specific sensitivity of individual organs to ultrasound, threshold exposure levels for biologic effects, cumulative dose-effects from multiple exposures, the effect of patient size and stage of gestation, is at best, minimally available. Ultrasound is a mechanical form of radiation with direct physical/force effects on biological tissues as well as indirect thermal effects from tissue energy absorption. High intensity ultrasound has well defined bioeffects and biohazards; but except for our data obtained over the preceding three years, the relationship between in vitro maximum values of ultrasonic field quantities measured and in vivo exposure levels in diagnostic studies remains unknown. We have constructed a specialized in vivo exposimetry system, developed and tested customized software and using specially fabricated hydrophones, determined selected first-order and second-order ultrasonic field quantities during a routine reproductive ultrasound examination of the ovary, embryo and fetus. Our sonographic measurements in the "ovarian group" yielded mean ultrasound beam path distances of 7.6 cm. in the presence of a distended bladder and 7.0 cm. in the presence of an empty bladder with an average group insertion loss of 6.2 dB and 7.3 dB; similar values for the "embryo group" were 8.35 cm. and 6.36 cm. respectively with an average group insertion loss of 5.7 dB and 9.9 dB; and for the "fetal group" were 9.6 cm. and 8.0 cm. respectively with an average group insertion loss of 8.7 dB and 8.9 dB. Using a Fixed Attenuation Model, the values were 2.98 dB/MHz, 2.71 dB/MHz and 4.17 dB/MHz for the ovarian, embryo and fetal studies respectively; whereas for the Overlying Tissue Model the respective values were 0.72 dB/cm-MHz, 0.65 dB/cm-MHz and 0.59 dB/cm-MHz. These data are both specific and unique in that they have been systematically obtained in situ. Although it is reassuring to note that these experimentally obtained human in situ data are generally consistent with some of the mathematical models proposed in the past, there are several major differences noted with respect to worst-case modelling and the fundamental reasons as to why there are similarities and differences between groups and bladder state conditions are not yet unknown. Using our now established methodology of placing the specially fabricated hydrophones in the uterine cavity of appropriate first trimester (embryo) and second trimester (fetal) pregnant patients and customized instrumentation, we will insonate the products of conception using commercial diagnostic ultrasound systems and: 1. Determine the maximum values of ultrasonic quantities to which the human embryo and fetus are exposed during a pulsed Doppler (including colorflow Doppler) ultrasound examination, 2. Use our experimentally obtained in situ data to evaluate the validity and accuracy of the Overlying Tissue MOdel versus the Fixed Attenuation Model; 3. Develop a quantitative five layer tissue model in humans to determine the coefficient of attenuation when an ultrasound beam passes through each of the following maternal tissues: (i) skin, (ii) subcutaneous fat; (iii) skeletal muscle, (iv) fascia, and (v) myometrium. We thus propose to establish a standardized and experimentally obtained database which can then be used as a reference for all future studies regarding the bioeffects of diagnostic ultrasound.